At a briefing at Nasa's Washington headquarters on Tuesday, Nasa scientists said the rock contains clay minerals that formed in a watery environment, an environment that may have been favourable for microscopic organisms.

Curiosity had already found a hint of the site's watery past, an ancient streambed that the six-wheel rover crossed to get to the flat bedrock.

The rover made a dramatic landing near the Mars equator last August for a two-year mission.